Infrared imaging is widely used in a variety of applications including night vision, surveillance, search and rescue, remote sensing, and preventive maintenance, to name a few. Imaging devices to provide these applications are typically constructed of HgCdTe or InSb focal plane arrays. These focal plane arrays are known to be pixel mapped devices, where an array element is generally mapped to one or more circuit elements. However, such focal plane arrays are difficult to manufacture and expensive. Quantum well infrared photodetectors (QWIPs) can detect mid and far infrared light, providing an output current as a result. However, such devices have not been able to be successfully used in efficient and inexpensive arrays to provide a practical imaging detector. QWIP devices are described in U.S. Pat. No. 4,873,555, issued Oct. 10, 1989 to the University of Pittsburgh and in U.S. Pat. No. 4,894,526 issued Jan. 16, 1990 to American Telephone and Telegraph Company, AT&T Bell Laboratories. The latter patent describes an improved efficiency device, which utilizes a series of quantum wells.
An improvement on these earlier technologies was disclosed by the applicant, H. C. Liu, in U.S. Pat. No. 5,567,955 issued Oct. 22, 1996 to the National Research Council of Canada, incorporated herein by reference, wherein the vertical integration of a light emitting diode (LED) with a QWIP is described. Current from the QWIP device resulting from the impingement of far-infrared (FIR) causing the LED to emit near-infrared (NIR) energy. This energy can be efficiently detected by a silicon CCD, resulting in a highly efficient detector. In the aforementioned patent, the applicant describes a FIR to NIR energy converter comprised of a QWIP photodetector integrated vertically with a LED. The integration results from epitaxial deposition of the LED material over the QWIP materials. The device described by the applicant in this earlier U.S. Pat. No. 5,567,955 relates to a transmissive device having a substantially transparent substrate.
However, in certain applications that will be described hereafter, it may be preferable to provide a device wherein the input FIR energy is launched into a same side (face) of the device as the up-converted NIR energy exits. More specifically, in a device, in accordance with this invention, wherein a large single QWIP-LED is provided, to be used as a pixelless means of up-converting and imaging a FIR beam to a NIR beam, means or measures must be provided to insure that unwanted distortion, smearing or shadowing of the image do not occur. Typical, substrates that are sufficiently thin to allow FIR energy to pass therethrough have been in the range of 300 .mu.m to 1 mm. However, providing a large single QWIP-LED in accordance with this invention, as a transmission device and using a substrate having a thickness as great as 300 .mu.m or greater, will result in images that are not absent of smearing and shadowing. Therefore, it is an object of this invention to provide a large QWIP-LED for use in up-converting and imaging input energy having a very thin substrate that will not cause smearing or shadowing.
It is an object of the invention, to provide a device wherein input FIR energy is launched into a same face as up-converted NIR energy exits.
It is a further object of the invention to provide a device that is less susceptible to blurring or smearing effects caused by using a standard transmissive substrate.
In U.S. Pat. No. 5,567,955, the applicant, H. C. Liu describes system that is "pixelized" wherein a plurality of QWIP-LEDs are fabricated on a single device, adjacent one another. The QWIP-LED elements are shown in the form of an array of closely spaced elements. In operation the energy in FIR wavelength to be detected passes through the transparent substrate 3 and is detected by each sub-QWIP exposed to the energy. The resulting photocurrents pass through the associated sub-LEDs causing them to emit energy of e.g. NIR. This NIR energy is detected by the CCD, and is processed for display in a well-known manner. In this prior art device, each QWIP-LED provides a single pixel of information to one or more CCD detectors. Although this device performs its intended function, it was found to be complicated, difficult manufacture, and costly. It is therefore an object of this invention to provide a device wherein a single QWIP-LED provides a pixelless image to a detection means such as an array of CCD elements capable of distinguishing and capturing a plurality of values from a single QWIP-LED. Thus, a novel aspect of this invention is the provision of a QWIP-LED, that is large enough to provide a pixelless beam for transporting an image to a capture means capable of recording a plurality of different detected values representative of the image within the beam. The invention also provides means of designing the QWIP-LED such that blurring and ghosting are minimized.
Methods for making GaAs-based QWIPs are well known, and have been taught in the cited patents. U.S. Pat. No. 4,873,555 teaches the basic methods for making a single-well detector, and introduces the new idea of using intraband or intersubband transitions for IR detection. U.S. Pat. No. 4,894,526 deals with a multiple quantum well detector. The details of the physics of QWIPs are found in the review article by Liu published as a book chapter in "Long Wavelength Infrared Detectors" edited by Razeghi. GaAs-based LEDs and Si CCD are widely available commercial products, and the basic physics and operation of these devices are discussed in standard text books, for example, in Secs. 12.3 and 7.4 of "Physics of Semiconductor Devices" by Sze.
Recently, a GaAs-based QWIP operating at a peak wavelength of 9 .mu.m has been integrated with a GaAs-based LED in the near infrared region, as published by Liu et al. in Electron. Lett., Vol. 31, pp. 832-833, 1995. The single element device described in this paper is functionally an IR to NIR up-converter. The incident IR causes an increase in the NIR emission intensity. The operation principle of this device is summarized below. A conventional QWIP works as a photoconductor, i.e., its resistance changes (usually decreases) when IR light of appropriate wavelength is launched onto the QWIP. A bias voltage is needed so that the QWIP operates at its optimum detection point. A standard LED emits light when biased close or beyond the flat band condition. Modem III-V epitaxial growth techniques such as GaAs-based molecular beam epitaxy (MBE) can grow both a QWIP and a LED onto the same wafer in a single stack. If one stacks a QWIP and a LED together by growth, one achieves a serially connected QWIP-LED. Applying a forward bias to this serial device results in turning both the QWIP and the LED into their operating conditions.
Prior art FIG. 1 illustrates in schematic a QWIP 1, having a series of quantum wells as described in the aforenoted U.S. Pat. No. 4,894,526, epitaxially grown on a substrate 3. An LED 5 is epitaxially grown on the QWIP. The substrate 3 is sufficiently thin and sufficiently transparent (i.e. 500--.mu.m) so as to allow FIR energy to pass through it to the QWIP 1.
It should be noted that the term FIR energy is intended in this specification to include mid infrared energy (MIR) of 3-12 .mu.m wavelength. FIR and MIR thus is used interchangeably in this specification. The NIR wavelength is approximately 800-1000 nm. Upon application of a bias current, e.g. from a battery 7 via a load resistor 9, to the stacked QWIP and LED in series, the same current passes through both. FIR generates photocurrent in the QWIP device, which passes into the LED.
This photocurrent arises from the external bias current, since the QWIP device changes resistance (usually decreases) when infrared light of the appropriate wavelength is launched onto the QWIP. This photocurrent generates or increases NIR emission from the LED, which emission can be detected by a detector that completes the thermal imaging device.
Absorption of infrared light at .lambda..sub.MIR results in an increase of the current flow in the QWIP device. This increase in current must increase the current through the LED since they are serially connected. Because the LED is biased near or beyond the flat band condition the additional current will give rise to turning on or increasing the LED near-infrared light emission. The mid-infrared light has thus been converted into a near-infrared light, which can be detected by a Si CCD.
Prior art FIG. 2 shows a cross section of a QWIP device and LED subdivided into an array of sub-QWIPs 11 and sub-LEDs 13, epitaxially deposited on a substrate forming mesas each of which is comprised of a sub-QWIP and sub-LED.